Thin film optical coatings of the interference filter variety consist of alternating layers of a low refractive index material such as silica and a high refractive index material such as tantala, titania, niobia and the like for selectively reflecting and transmitting different portions of the electromagnetic spectrum emitted by the filament. In one application, these coatings form an optical interference filter for selectively reflecting infrared energy emitted by the lamp filament back to the filament while transmitting the visible light portion of the electromagnetic spectrum emitted by the filament. As a result, the amount of electrical energy required to maintain the operating temperature of the filament is reduced.
Such filters and lamps employing same are disclosed, for example, in U.S. Pat. No. 4,229,066, which issued to Rancourt et al on Oct. 21, 1980. Rancourt et al disclose a visible transmitting and infrared reflecting filter comprising at least one period of a stack formed of a plurality of layers of high and low index materials with alternate layers being formed of materials having a high index of refraction and the other layers being formed of materials having a low index of refraction. One or more anti-reflection layers are disposed between adjacent layers of the period. The forty-seven layer filter presented as an example in column 5 of Rancourt et al includes thirty-seven thin layers (i.e., less than 400 Angstrom).
U.S. Pat. No. 5,138,219, which issued to Krisl et al on Aug. 11, 1992, discloses a filter comprising three spectrally adjacent multiperiod stacks with the first stack being a conventional shortwave pass stack having at least two periods. The second and third stacks are spectrally located at wavelengths different from each other and longer than the wavelength of the first stack. The latter two stacks each comprise at least two periods, with each period containing seven alternating layers of high and low refractive index materials. In the example presented in TABLE 2, Krisl et al disclose a filter containing 24 thin layers to provide a spectrally broad high transmittance between 400 and 770 nanometers.
Although the above-described filters of Rancourt et al and Krisl et al may be effective for reflecting infrared radiation while transmitting visible radiation, certain disadvantages still exist. For example, it is well known that thin layers of such filters are more difficult to deposit than thick layers.
U.S. Pat. No. 4,663,557, which issued to Martin, Jr. et al on May 5, 1987, discloses a filter comprising three spectrally adjacent multiperiod stacks with the first and third stacks being conventional shortwave pass stacks and the second stack being a 2:1 dielectric stack. While all of the layers of the filter are relatively thick (i.e., greater than 400 nanometers) and therefore much easier to deposit, the produced filter is more sensitive to errors in deposition because the window of visible transmission is very narrow.
Often optical coatings of the prior art are designed for maximum reflection over the infrared and maximum transmission over the visible. However, this design criteria ignores the fact that a lamp is a Plankian radiator and thus does not have a flat radiated energy distribution.